The CDM session was being destroyed before the policy timer was stopped
and before the session was removed from a list of active sessions. This
allowed race conditions, where the policy timer would try to evaluate
policy for a closed session. This led to segfaults.
b/11338324
Merge of https://widevine-internal-review.googlesource.com/#/c/8240/1
from the widevine cdm repo.
Change-Id: Ib159ccfdb763a47da573f5c06c0793c2c63886c4
49e593d127